1. Field of the Invention
The present invention relates to a liquid crystal display device utilizing a liquid crystal panel capable of forming an optical image as a function of change in a light scattering condition, and also to a liquid crystal projection device for projecting the optical image formed on the liquid crystal panel onto a screen to provide the image of an enlarged scale.
2. Description of the Prior Art
A large-format image presentation such as, for example, in a home theater or in a panel discussion has now come to be popular. While projection devices utilizing a light valve have been available in various types, a recent development is a liquid crystal projection device operable to project an image, formed on a compact liquid crystal panel, onto a screen to thereby provide a large-format image.
The liquid crystal panel provides an image display by essentially electrically varying an optical characteristic of the liquid crystal panel and operates on a number of principles. The twisted nematic (TN) liquid crystal panel generally employed in the liquid crystal projection device currently available in the market makes use of a phenomenon in which the rotatory polarization of liquid crystal material varies in the present of an electric field. This TN liquid crystal panel requires the use of two polarizing plates to be disposed on or adjacent opposite surfaces, that is, light incident and exit surfaces, of the liquid crystal panel and, for this reason, the TN liquid crystal panel has a problem in that the efficiency of utilization of light is low.
On the other hand, as a method of controlling light with no polarizing plate used, a method of utilizing a scattering phenomenon is known. The liquid crystal panel capable of forming an optical image by the utilization of changes in a light scattering condition is available in three models which utilize a phase-changeable liquid crystal material, a dynamic scattering liquid crystal material and a polymer dispersed liquid crystal material, respectively. Of them, the polymer dispersed liquid crystal panel such as disclosed in the U.S. Pat. No. 4,435,047 is extensively studied because it gives rise to a bright image display.
The display panel of a type utilizing the polymer dispersed liquid crystal material neither requires the use of any polarizing plate nor any orientation treatment. In the TN liquid crystal display panel, light lost by the polarizing plates are absorbed by the polarizing plates and converted into heat which would eventually elevate the temperature of not only the polarizing plates, but also the liquid crystal panel by radiation. Accordingly, once the polarizing plates and the liquid crystal panel are so heated, they quickly deteriorate. Also, the TN liquid crystal panel requires formation of an orientation film which must subsequently be rubbed. This rubbing constitutes not only a cause of increase of the number of process steps, but also a cause of reduction in yield, because thin-film transistors are destroyed by static electricity, resulting in en increase of the manufacturing cost. Also, the liquid crystal panel currently employed in the liquid crystal projection television set employs a large number of pixels, for example, 300,000 pixels or more, and consequently, attempts have been made to reduce the size of each pixel. A reduction in the size of each pixel requires the formation of an increased number of signal lines and thin-film transistors, accompanied by the formation of an increased number of surface indentations, which in turn hampers a smooth rubbing treatment.
Hereinafter, the nature of the polymer dispersed liquid crystal material will be briefly discussed. The polymer dispersed liquid crystal material may be broadly classified in two types depending on the type of liquid crystal and the condition in which polymer molecules are dispersed. One type is that droplets of liquid crystal are dispersed in the polymer and exist in the polymer in a discontinuous fashion. This type is hereinafter referred to as PDLC (polymer dispersed liquid crystal). The other type is polluter network liquid crystal (PNLC) in which a network of polymer molecules is formed in a layer of liquid crystal material as if the liquid crystal material is soaked in a sponge. In this PNLC type, the liquid crystal exists in a continuous fashion without forming droplets. In order for the liquid crystal display panels utilizing the polymer dispersed liquid crystal material and the polymer network liquid crystal material, respectively, to accomplish an image display, scattering and transmission of light must be controlled.
For the purpose of discussion of the present invention, the term PDLC (polymer dispersed liquid crystal) herein used is to be understood as including not only the polymer dispersed liquid crystal material, but also the polymer network liquid crystal material as well.
As a polymer matrix used in the polymer dispersed liquid crystal layer, any thermoplastic and thermosetting resin may be employed if and only if it is transparent, but a UV-curable resin is mostly employed therefor because of its excellent performance and because the liquid crystal panel can be manufactured using a method generally employed for the manufacture of the conventional liquid crystal panel. According to the popular method of making the conventional TN liquid crystal panel, a predetermined electrode pattern is formed on each of upper and lower substrates and these two substrates are subsequently overlapped with each other with the electrodes in one substrate aligned with those in the other substrate. At this time, spacers having a predetermined uniform size are sandwiched between these two substrates to keep the substrates spaced apart from each other by means of a sealing member of epoxy resin, thereby defining a liquid crystal chamber therebetween. Thereafter, a quantity of liquid crystal material is injected into the liquid crystal chamber between the substrates.
In order for the PDLC panel to be manufactured by the use of the above discussed known method generally used to manufacture the conventional TN liquid crystal panel, if a UV-curable resin, for example, a UV-curable acrylic resin, is employed as a polymer matrix, the resin exists in the form of a precursor of a relatively low viscosity such as a monomer and/or an oligomer and, since a blend of the UV-curable resin with the liquid crystal material (hereinafter referred to as a liquid crystal solution) has a fluidity sufficient to allow it to be injected at normal temperatures, and if the UV-curable resin is cured by UV radiation after the liquid crystal solution has been injected by the utilization of the conventional method of making the TN liquid crystal panel, to thereby phase separate only the liquid crystal material to form a polymer dispersed liquid crystal layer, the liquid crystal panel of a dispersed type can easily be formed.
The operation of the polymer dispersed liquid crystal will be briefly discussed with reference to FIGS. 18A and 18B, which are explanatory diagrams used to explain the operation of the polymer dispersed liquid crystal panel. In these figures, reference numeral 181 represents an array substrate; reference numeral 182 represents a pixel electrode; reference numeral 183 represents a counterelectrode; reference numeral 184 represents liquid crystal droplets; reference numeral 185 represents a polymer; and reference numeral 186 represents a counterelectrode substrate. The pixel electrode 182 is connected with a thin-film transistor (not shown) and others, and a voltage is applied to the pixel electrode by switching the thin-film transistor on and off to vary the direction of orientation of the liquid crystal material on the pixel electrode to thereby modulate light. When and so long as no voltage is applied as shown in FIG. 18A, the liquid crystal droplets 184 assume a random orientation. In this condition, a difference is created between the refractive index of the polymer 185 and that of the liquid crystal droplets 184, and the incident light is therefore scattered. Application of the voltage to the pixel electrode 182 results in uniform alignment of the liquid crystal material as shown in FIG. 18B. If the refractive index of the liquid crystal material when the latter is oriented in one predetermined direction is chosen to be equal to that of the polymer, the incident light emerges outwardly from the array substrate 181 without being scattered. It is to be noted that, when the liquid crystal material is expressed in the form of droplets such as exhibited by PDLC, the mean value of respective diameters of the liquid crystal droplets is referred to as the average particle size.
In order to convert the optical image, formed on the liquid crystal panel as a change in light scattering condition, into a change in luminance, a phenomenon is utilized in which, when only a portion of the light emerging outwardly from the liquid crystal panel, which portion is encompassed by a predetermined solid angle, is extracted, the amount of light encompassed by such predetermined solid angle varies depending on the light scattering condition. In general, a system is widely used in which light travelling towards a center of orientation by the use of an aperture is utilized. In other words, if the light scattering power increases, the amount of light emerging from the liquid crystal panel and incident on a projection lens decreases. Although this aperture type has a relatively simple structure and can provide a bright projected image, there is a problem in that the contrast ratio is not satisfactory. As one method to increase the contrast ratio, it is contemplated to reduce the solid angle in which the projection lens collects light, but this in turn reduces the brightness of the projected image.
Where the display panel is used under a bright environment, a reduction in contrast is considerable when affected by external light and the displayed image will no longer be recognized. In the case of the TN liquid crystal panel which is featured in that polarized light is emitted therefrom, use has been made of a polarizing screen to minimize the reduction in contrast which would occur when affected by the external light, so that the displayed image can be sufficiently recognized even under a bright environment. Where the displayed image is to be projected onto the polarizing screen, the polarizing plate disposed on the exit side of the TN liquid crystal panel must have an axis of polarization held in alignment with that of the polarizing screen. Unless the respective axes of polarization are aligned with each other, the displayed image will be darkened. Once the axis of polarization of the polarizing plate of the TN liquid crystal panel and that of the polarizing screen have been fixed, it can no longer be changed. On the other hand, since light projected by a liquid crystal projection device which utilized as a light valve the liquid crystal panel capable of forming an optical image as a function of change in light scattering condition is natural light, the influence brought about by external light cannot be suppressed even though the polarizing screen is employed.
Where projection of an image is made in a place rich in external light, the TN liquid crystal panel is effective to suppress the reduction in contrast if the polarizing screen is employed. However, where projection of an image is made in a place substantially free from external light, the feature of the polarizing screen cannot be best utilized, and the brightness will be so insufficient that projection of a large-format image cannot be accomplished.
The TN liquid crystal panel has additional problems associated with orientation and polarization. If orientation is irregular in the TN liquid crystal panel, linearly polarized light which has passed through the polarizing plate on the incident side of the TN liquid crystal panel will not be rotated 90.degree. as it passes through the liquid crystal panel accompanied by a reduction in transmittance at the polarizing plate on the exit side of the liquid crystal panel and/or light leakage resulting from a reverse domain generated under the influence of a transverse electric field developed between the signal lines and the pixel electrodes.